This invention relates to the use of a binary lead-tin alloy substrate as the current collector and mechanical support for the electrochemically active material in pasted electrodes (Faure type) for lead-acid cells particularly of the normally sealed type.
Traditionally, the grid substrate material in
lead-acid cells has been either pure lead (minimum 99.9 percent by weight) or a lead alloy containing metals such as calcium and antimony. Pure lead, which provides extended float life through reduced corrosion, is oftentimes too soft to be used or processed as a grid material except for special applications such as in Plante plates and some spirally wound batteries. To stiffen the grid material, calcium and antimony are commonly alloyed with lead. Tin has also been added to alloys of lead. Tin, when added to alloys of lead-calcium operates synergistically with the calcium to improve metal fluidity and castability. Tin in the amount of 0.5 to 1.0 weight percent added to alloys of lead-calcium also enhances the mechanical and corrosion resistant properties of the alloy. Minimal amounts of tin are used because of the expense and relative scarcity of the element.
Passivation in the positive plate of lead-acid cells is a problem that has been known for some time. Passivation has been attributed to the formation of lead oxide (tetragonal PbO) at the grid/active material interface in the positive plate. Lead oxide acts effectively as an insulator on the grid leading to passivation. The result is increased impedance within the lead-acid cell which is manifest as poor recovery from deep discharge. This refers to the lead-acid cell's inability to recover capacity when the cell is allowed to stand after complete discharge and is subsequently charged.
In dry charge plates either prepared in a Plante L or Faure type process, tin reduces the formation of the passivation layer on the grid material while in storage. The effect is that dry charge plates containing tin maintain high activation rates even after a long, dry run storage. In Plante-type plates, use of a binary lead-tin alloy containing less than 1 weight percent tin is known. In Faure-type dry charged plates, the "tin effect" is not exhibited until the concentration of tin is greater than 0.5 weight percent and when alloyed with other metals such as calcium.
In normally sealed lead-acid cells, tin has been incorporated in the grid material to prevent passivation of the positive electrode. The addition of tin has been accomplished by simple alloying and by deposition of a tin-rich layer on the surface of the grid by such methods as lamination. To facilitate recovery from deep discharge, Kobayashi et.al. U.S. Pat. No. 4,761,356 teaches that the content of tin should exceed three weight percent.
Lead-tin alloys have also been used as material for both intra- and intercell connectors and the positive posts of a lead-acid batteries. In a connector, it has been disclosed that the tin content should be less than ten weight percent to maintain the formability of the connector. A tin content between 0.1 and 1.5 weight percent has been used in the positive posts. Such posts are disclosed to have the advantage of good adherence, promoting long lifetime of the post seals attached to the post.
It is a primary objective of this invention to provide a substrate material to be used as the current collector and mechanical support for the active material in lead-acid cells which leads to enhanced performance by improving the ability of the cell to recover from deep discharge and to increase the cold cranking ampere capacity.
It is another object of this invention to provide an electrode for a lead-acid cell which is not subject to passivation while minimizing cost by reducing the amount of tin used in the electrode well below the three weight percent minimum level taught by the prior art before the effects of tin are realized.
It is a further object of this invention to provide a substrate material in which the tin is available throughout the lead sheet and in particular throughout the grain body of the alloy to ensure that there are active controlled corrosion sites continually available to inhibit the formation of the passivation later at the boundary between the substrate material and the active material.